Poly axial screws are used e.g. for fixation of a patient's spinal column in spinal surgery that is performed to fuse individual spinal vertebrae. A screw shaft is inserted into the bone, e.g. the pedicle of each vertebrae to be fused, and a fusion rod is attached by connectors mounted on the screw shaft to multiple screws, to form a supporting structure that fixates the vertebrae and prevents movement. Over time, lack of movement as well as bone transplants implanted at the surgery site for this purpose allows bone to grow and permanently fuse the bones of the vertebrae after surgery.
Many different models exist for pedicle screws which generally have a screw shaft and a U-shaped connector that connects the screw to a fusion rod. In mono axial screws, the head of the screw can be aligned with the position of the rod only to a limited extent, necessitating complicated bending to correct the angle. Poly axial screws generally have a screw shaft with ball head and the U-shaped connector or tulip rotatably and pivotably coupled thereto.
While poly axial screws allow more flexibility in positioning of the screw shafts and fusion rods, they often lack the rigidity or stability required to achieve and maintain an accurate angular placement during surgery, may be difficult to connect to a fusion rod, and/or create even more bulky structures that interfere with surgery, causing more tissue damage and/or require larger incisions.
Typically, the parts of a poly axial screw are configured for pre-assembly before implantation, the shape, size or number of their parts making them complicated to assemble, bulky, and/or are difficult to implant. The resulting system may lack flexibility or rigidity or both. The bulk of the poly axial screw system can also complicate surgery before and after implanting of the screws, as it may require larger or different incisions and more soft tissue may have to be removed, thus complicating surgery and prolonging healing times. Also the bulky structure, once implanted, may interfere with the view of the surgeon during the ongoing surgery, e.g. during placement of bone implants, and also if discs (between the vertebrae) are removed and replaced with interbodies, as is often the case (e.g. herniated discs). Further the bulk may interfere with imaging techniques to determine correct placement of the screw system itself, or of the placement of related therapeutic components, such as interbody transplants and positioning tools.
In some systems, threaded fasteners are used to secure the rods to the connectors. The rods can be provisionally held in position by loosely tightening the fasteners, and after desired adjustments are made, the fasteners can be further tightened by torque and fixated into a final position. Such procedures typically requires a torque wrench or similar device, and to prevent torque from being transferred to the patient, require an anti-torque device in parallel. The effective use of both devices requires skill and is dependent upon the strength and experience of the surgeon. The provisionally loosely tightening typically results in unpredictable inconsistent results with regards to adjustability—the fastener may have been tightened to little, resulting in the correct position being lost, e.g. as the structure shifts in the course of the ongoing surgery as the structure or the surrounding bone and tissue is manipulated, or the fastener may have been tightened too much, so adjustments cannot be made without first loosening it again, applying unnecessary torque or requiring use of an anti-torque device.
Therefore, there is a need in the art for a poly axial screw system that avoids bulk, is easily and quickly assembled, has flexible assembly options, is easily positioned, provides a sufficient and consistent degree of rigidity to retain its preliminary axial position after assembly, and is easily and quickly adjusted before fixation into the final position. Also, there is a need in the art for a system that avoids excessive torque, in particular when adjusting repeatedly before fixation. These and other features and advantages of the present invention will be explained and become apparent to one skilled in the art through the following description of the invention.